Post tax contribution

A plan participant was able to write a check to his plan account on his own.  Not deferral, not through payroll, etc.

The plan does not allow for post tax contributions.  How to correct?

Give the money back net of losses, if any.

And as long as the return of the money occurs no later than the end of the second plan year after it went it, should qualify as self-correction of operational error under EPCRS.

And even if not by the end of the second plan year it might very well be an insignificant error allowing self-correction under EPCRS.

Right. Probably insignificant in any event, I agree.